Atomic and molecular quantum dynamics

Introduction

Our experimental work focuses on the quantum dynamics of simple ionic systems ranging from
atoms to cold molecules and clusters. This research has direct impact on the field of
quantum chemistry and on basic few-body quantum physics regarding the dynamics of systems
including several particles in highly excited or strongly correlated motion. The results
contribute important experimental benchmarks for molecular reactions in the cold
interstellar medium as well as for atomic processes in hot, highly ionized astrophysical
surroundings, for basic atomic and molecular theory, chemical physics involving ions, and fundamental physics such as
quantum electrodynamics. The research is performed with the
ion storage ring TSR,
in operation until end of 2012, the cryogenic ion storage ring CSR nearing completion,
cryogenic radiofrequency ion traps and
ultracold electron beams and covers a broad scope
of atomic and molecular processes.

Dissociative electron-molecule recombination and related processes

Dissociative recombination is an important reaction of ionic molecular compounds with
free electrons, which has no energetic barrier and works also at the lowest temperatures.
It leads to the destruction and chemical conversion of the molecules and produces further
chemically active radicals as fragments. Predicting the rates and the product channels for
these reactions requires detailed knowledge of inner-molecular dynamics actively studied
worldwide both experimentally and theoretically.

In our experiments we combine translationally and vibrationally cold molecular ion
beams at the ion storage rings TSR and
CSR
with continuously improved techniques
of event-by-event fragment imaging,
suitable even for neutral
products and for multi-coincidence events from polyatomic systems, in order to reveal the
inner mechanisms of molecular dissociation. Stored ion beams offer the unique opportunity
to thermalize the internal molecular degrees of freedom (vibration and, partly, rotation)
with the surrounding storage environment, defined by the blackbody field in the vacuum
enclosure as well as by the electron beams interacting with the stored ions. New
ultracold electron beams
from cryogenic photocathode sources are applied for electron collision studies with
record-high impact energy resolution.
Recent studies aim in particular at polyatomic molecular ions important in ion
chemistry, as in astrophysical and atmospherical processes, and the understanding of
the basic mechanisms.

Cold radiofrequency ion traps and laser spectroscopy

Molecular ions with low internal temperatures are produced and investigated in
a cryogenic radiofrequency ion trap suitable for buffer gas
cooling of the ions down to about 10 K. This trapping technique was implemented
at the TSR to inject pre-cooled molecular ions, in particular H3+, a main species
driving chemical reactions in cold dilute media, notably in astrophysics; the
project triggered the development of cold ion injectors at various ion storage
devices worldwide. It is used to study at the TSR the
dissociative recombination of H3+ in the lowest quantum level of
the ortho and para nuclear spin variants of H3+, respectively. Moreover,
highly sensitive ro-vibrational laser spectroscopy of H3+
is performed in the ion trap itself.
With the new methods, precise H3+ laser spectroscopy could be performed under dilute,
ion trap conditions. Techniques even extending to ion beam environments are in
development.

Coulomb explosion of molecular ions with energies around 1 MeV, either from the
TSR or directly from the accelerators of the institute, can be initiated by passing
them through very thin foils where they lose their binding electrons on a
sub-femtosecond time scale. The event-by-event imaging of the fragmentation
products under these conditions takes snapshots of the molecular vibrational
motion and allows important parameters, such as the binding length and the
vibrational force constants reflecting the molecular potentials, to be determined.
Experiments were also performed on the negative hydrogen molecules
(in particular H2-), which are transient species living shorter than a millisecond
and become relatively stable only if they strongly rotate around their axis.
In recent studies the sense of cirality of a partially deuterated epoxide sample could
be determined directly by this molecular imaging method.

In a plasma, the internal degrees of freedom (rotational, vibrational or electronic)
of molecular and cluster ions are often highly excited by collisons with other particles.
The fastest channels for relaxation of these states are the emission of an electron or an
atomic fragment. For bulk matter these processes are known as thermionic electron emission
and evaporation.
A particularly important aspect of these processes is the energetic coupling of
electronic and vibrational energy in these complex systems, having a large number of
oscillation modes very high densities of excited vibrational levels.
To achieve high sensititvity and a low radiation background
in these collision studies, beams of
small molecular and cluster
ions are stored
at high velocity in cryogenic storage devices. In pilot studies for the development
of the cryogenic storage ring (CSR), the
Cryogenic Trap for Fast ion beams (CTF)
has been developed and used for such experiments. The decay of complex negative ions
such as Al4- and SF6- was
examined
with particle counting and imaging detectors.
Read more on the pages of the CSR and
the CTF.

Resonant - dielectronic - recombination is the most important recombination
mechanism of highly charged ions in hot, dilute plasma. In particular for
photoionized plasma in astrophysical environment close to strong radiation
sources, large arrays of dielectronic resonances define the recombination
rates relevant for understanding the abundance of charge states.
This is similarly true for highly charged ions in hot terrestrial plasmas - such
as in fusion reactors. Important systems studied are highly charged systems of iron
(such as Fe7+ to Fe17+) and tungsten (W20+).
Predicting
recombination and also electron impact ionization rates for these multielectron
systems requires a wide range of approximations in order to become tractable within
the present capabilities of theoretical physics. The predictions used in
astrophysical models are submitted to stringent benchmark tests by measurements
in the TSR with highly charged ion beams and ultracold electron beams.

Excitation energies of highly charged ions, measured with high precision, reflect
the rich dynamics of virtual particle pairs in strong Coulomb fields, described
by quantum electrodynamics. Here, these quantum-electrodynamical interactions
are particularly studied in systems composed themselves of several charged particles
already. The relevant energy levels of the highly charged ions become accessible
through their resonant - "dielectronic" - recombination with electrons, investigated
with ion beams stored in the TSR and interacting with the ultracold electron beam
available there.

Lifetime measurements of metastable levels in stored atomic ions

Most atoms, even in their high charge states, can exist in states which remain
highly excited for extended periods of time, up to milliseconds or seconds, because
their internal symmetry prevents their decay which normally would occur in much
smaller fractions of a second by the emission of electromagnetic radiation. Such
metastable excited ions in high charge states are stored in the TSR and their
decay periods on the millisecond to second time scale are monitored and
precisely measured.
Read more … >